1. Field of the Invention
The present invention relates to a vibration-isolating device for preventing vibrations due to ground motions or running of an automobile from being propagated to a platform which is disposed in, for example, a semiconductor device factory or a laser device factory, and on which various kinds of equipment are to be mounted, and also from causing the platform to vibrate. More particularly, the present invention relates to an active vibration-isolating device which uses a piezoelectric actuator and in which the driving of the piezoelectric actuator can be externally controlled so as to interfere with vibrations of a platform, thereby eliminating excitation vibrations or vibrations in a low-frequency region of equipment mounted on the platform.
2. Description of the Prior Art
Conventionally, an active vibration-isolating device of this kind includes a device as disclosed in Laid-Open Japanese Utility Model Publication No. 6-58254. The active vibration-isolating device disclosed in the publication basically comprises a piezoelectric actuator, and a mechanism which transmits the driving displacement of the piezoelectric actuator via two piston members having different diameters and a working fluid. A rubber elastic member is adherently interposed between one or both of the two piston members having different diameters and a housing member which surrounds the piston member and has a side face that is substantially parallel with that of the piston member.
In the prior art active vibration-isolating device configured as described above, the rubber elastic member interposed between the two piston members and the housing member prevents the working fluid from leaking. Therefore, there is no friction between the piston members and the housing member, so that the piston members operate smoothly. As a result, the driving displacement of the piezoelectric actuator is efficiently transmitted via the two piston members and the working fluid, thereby enabling predetermined vibration-isolating performance and vibration-damping performance to be surely exerted.
In the prior art active vibration-isolating device, also the rubber elastic member interposed between the two piston members and the housing member shares the load of equipment mounted on the device. In the case where mounted equipment exerts a large load and the rubber elastic member must support a large shared load, such as the case where the device is disposed in a semiconductor device factory, a laser device factory, or the like, therefore, the rubber elastic member adherently interposed between the smaller-diameter piston member and the housing member is distorted and deteriorates. In order to prevent such distortion and deterioration of the rubber elastic member from occurring, the rubber elastic member must have a large spring rate. As a result, there arises a problem in that the resonance frequency is raised and the vibration-isolating performance in a low-frequency region is impaired. Furthermore, the preload to be applied to the piezoelectric actuator is increased, and hence there arises also a problem in that the driving efficiency of the piezoelectric actuator is lowered.